For high quality sewing of stitch seams utilizing high-speed sewing machines, especially for the formation of stitch seams on visible portions of high quality garments, it is important that the stitch terminate at particular locations and neither overrun nor underrun the desired end point of the stitch seam significantly.
This is necessary so that the stitch seam appears, as much as possible, to be similar to a handstitched seam.
For example, when pocket flaps are sewn on an outer garment, e.g. jackets, trousers, dresses or the like, the flap is generally stitched to the garment at its upper edge by a stitch seam whose length is precisely determined by the length of the flap.
The exact point which the stitch seam should commence is readily determined since the flap is generally brought into contact with the sewing machine needle in a lowered position thereof or into contact with an abutment to precisely position the flap vis-a-vis the sewing location.
At the opposite end of the seam, however, such positioning is not possible.
With manual operation of a sewing machine, the operator, as he approaches the end of the stitch seam, visually determines the point at which the seam should end and generally slows down the rate of feed of the fabric and the speed of the machine so that the final stitch should be as close as possible to the visually determined end point.
It is possible in this manner, to shorten the last stitch or stitches to bring about a reasonably precise end to the stitch seam.
However, with automatic sewing machines and especially high-speed sewing machines in production plants for garments, a manual reduction in speed of the machine and indeed even operator control of the termination of the stitch seam is generally not possible without significantly reducing the production rate and rendering the automation of the plant a nullity.
Automatic sewing machine installations, therefore, must rely upon other means for determining the end of the flap and for terminating the formation of the stitch seam by cutting off the sewing machine at the desired point.
For this purpose, it is known to provide upstream of the stitching location in the direction of feed of the fabric work pieces, a reflective light curtain which responds to the passage of the end of the flap and operates a presettable stitch counter which counts down the number of stitches corresponding to the distance between the curtain and the stitching location, thereby terminating the operation of the sewing machine when the preset count is achieved.
In the case of the flap of a pocket or the like, the trailing edge of the flap is detected by the light curtain and only the number of preset stitches on the presettable stitch counter can be then generated by the machine.
While this system is effective to terminate the stitch seam approximately in the region of the trailing edge of the flap, it nevertheless is unsatisfactory when the stitch seam is to appear more or less as a manually sewn seam.
The reason for this is inherent in the machine structure and system.
For example, the arm of the sewing machine is provided with an arm shaft which drives the needle in its up-and-down motion, each revolution of the shaft corresponding to the formation of a complete stitch of the customary stitch length (hereinafter referred to as the single stitch length).
Since the arm shaft can assume an infinite number of angular positions at the point at which the light curtain signals the beginning of the countdown of the presettable stitch counter, and the counter will result in a complete preset number of stitches generated after it is triggered by the light curtain, the last stitch point seldom coincides with the ideal end point of the seam, but rather can overrun or underrun this set point by almost an entire stitch length.
In other words, the final position of the needle can be approximately a full stitch length ahead of or behind the ideal end point.
This is invariably the case because the feed of the fabric is also coupled to the arm shaft rotation.
Thus, in the case of long stitches, i.e. single-stitch lengths which are considerable, the overrun or underrun may be highly visible and will generally detract from the quality of the garment.